Following the last race of the 2005 season, nine of the ten Formula One teams refrained from testing until the end of November. The agreement on a testing 'ban' was meant, theoretically, to allow the teams to rest after the long slog of a 19-race season. In reality, the teams have had little time to rest.
The designers worked flat out on the new car; the production department was split between making the tooling and parts for both the test cars and the new car; and the mechanics were all back at the factory, building up the test cars in preparation for the dawn of the winter testing season.
How the teams tackle winter testing depends on the state of development of their old car: how much needs changing for the new car and also what rules have changed ahead of the upcoming season.
Some teams with greater resources will make interim cars for testing - or simply complete lots of mileage with an old car. At the other end of the pitlane, lesser-funded teams are lucky to get any testing done, with their new car not being ready until the last minute.